10 Answers
10

I think you could define the width of a border using units like em which would come out to less than 1px, and it would be valid. However, like Will Martin said, for display purposes it would just round it up or down to a whole pixel.

Edit: I have overseen then IE6 restriction, but I leave the answer here for others ...

Its possible with transform:scale(0.5) and put a border element with border:1px; inside. So you get a half pixel border, that (although tricked and browser dependend) is displayed on screen. But I use that trick for printing.

sure, you have to adapt the content of the element, or play with position

No. You can't show a size smaller than one pixel because pixels are the basic unit of the monitor. And anyway, no browser I know of allows you to specify sub-pixel widths. They just get rounded up to 1px or down to 0px.

With modern resolutions this is no longer true. Monitors can render sub-pixel quality display items and many fonts actually require it in order to look decent on a retina style display.
– Bjorn TiplingJul 9 '15 at 17:22

Also an issue for printing—Chrome 52 ignores the (probably unknowable for it) printer pixels and uses 1px = 1/72in as the thinnest possible line. Tried fractional pixels, inches, ems—none would create a line thinner than 1/72in. Even in a print media query. Tempted to see if using an image works, but I suspect I'll just make my peace with the chubby rules
– Roger KruegerAug 30 '16 at 3:03

I don't know about IE8-10 (IE6-7 definitily no go) , but in Chrome and FF I get the thinnest border with box-shadow. Works best to get a 1px <hr> instead of the autorendered 2px, but can be used on a border as well.

The thin border on the HR is more prominent in FF than Chrome, but also Chrome renders 2px.